1. Field of the Invention
The present invention relates generally to Hybrid Automatic Repeat reQuest (HARQ) feedback, and more specifically, to a HARQ acknowledgement feedback method and apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) radio communication system.
2. Description of the Related Art
Mobile communication systems have been developed to provide mobile subscribers with voice communication services. With the rapid advance of technologies, mobile communication systems have evolved to support high-speed data communication services as well as the standard voice communication services. The limited availability of resources, in addition to user requirements for services at higher speeds in the current mobile communication system is spurring progress to more advanced mobile communication system.
Long Term Evolution-Advanced (LTE-A) is a next generation mobile communication standard that is being developed in order to meet such user requirements. LTE-A is being standardized by the 3rd Generation Partnership Project (3GPP). LTE-A is a technology for realizing high speed packet-based communication at about up to 1 Gbps. In order to achieve LTE-A deployment, LTE-A developers are discussing several communication schemes, such as network multiplexing for deploying multiple overlapped evolved Node Bs (eNBs) in a specific area and increasing the number of frequency bands supported by each eNB.
Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique for transmitting data using multiple carriers (i.e., a multicarrier data transmission technique) that parallelizes a serial input stream into parallel data streams and modulates the parallel data streams onto orthogonal multiple carriers (i.e., sub-carrier channels).
Multicarrier modulation schemes originated in the late 1950's with the use of microwave radio for military communication purposes. OFDM using orthogonal overlapping multiple subcarriers has developed since the 1970's, but is limited in applications to real-world systems, due to the difficulty in implementing orthogonal modulations between multiple carriers. With the introduction of the idea of using a Discrete Fourier Transform (DFT) for implementing the generation and reception of OFDM signals in 1971, OFDM technology has rapidly developed. Additionally, the introduction of a guard interval at the start of each symbol and use of Cyclic Prefix (CP) overcomes the negative effects caused by multipath signals and delay spread.
Due to such technical advances, OFDM technology has been applied in various digital communications fields such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Wireless Local Area Network (WLAN), and Wireless Asynchronous Transfer Mode (WATM). The implementation complexity of OFDM has been reduced by the introduction of various digital signal processing technologies such as Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
OFDM is similar to Frequency Division Multiplexing (FDM), but OFDM is much more spectrally efficient in achieving high speed data transmissions by overlapping multiple subcarriers orthogonally. Due to the spectral efficiency and robustness to the multipath fading, OFDM has been considered as a prominent solution for broadband data communication systems.
OFDM is advantageous due to the ability to control the Inter-symbol Interference (ISI) using guard intervals and to reduce the complexity of equalizers in view of hardware as well as spectral efficiency and robustness to the frequency selective fading and multipath fading. OFDM is also robust to impulse noise when employed in various communication systems.
In wireless communications, high-speed, high-quality data services are generally hindered by channel environments. In wireless communications, channel environments suffer from frequent changes, not only due to additive white Gaussian noise (AWGN), but also due to power variations of received signals caused by a fading phenomenon, shadowing, a Doppler effect brought by movement of a User Equipment (UE), a frequent change in a velocity of the UE, interference by other users or multipath signals, etc. Therefore, in order to support high-speed, high-quality data services in wireless communication, there is a need to efficiently overcome the above channel quality degradation factors.
In OFDM, modulation signals are located in two-dimensional time-frequency resources. Resources on the time domain are divided into different OFDM symbols, and are orthogonal with each other. Resources on the frequency domain are divided into different tones, and are also orthogonal with each other. An OFDM scheme defines one minimum unit resource by designating a particular OFDM symbol on the time domain and a particular tone on the frequency domain, and the unit resource is called a Resource Element (RE). Since different REs are orthogonal with each other, signals transmitted on different REs can be received without causing interference to each other.
A physical channel is a channel defined on the physical layer for transmitting modulation symbols obtained by modulating one or more coded bit sequences. In an Orthogonal Frequency Division Multiple Access (OFDMA) system, a plurality of physical channels can be transmitted depending on the usage of the information sequence or receiver. The transmitter and receiver determine REs on which a physical channel is transmitted, and this process is called mapping.
LTE and LTE-A systems are representative systems adopted OFDM in downlink and Single Carrier-Frequency Division Multiple Access (SC-FDMA) in uplink.
Meanwhile, in an LTE Time Division Duplexing (TDD) system, each eNB's HARQ acknowledgement transmission timing corresponding to the data transmitted by a UE is determined at uplink transmission timing and transmitted at predetermined downlink subframes. In the LTE-A TDD system, however, it is necessary to allow transmission of the HARQ acknowledgement at all of the subframes to prepare the eNB traffic adaptively and support multicarrier transmission. Due to the backward compatibility problem of the legacy UE, it is impossible to transmit HARQ acknowledgement designed for legacy systems at subframes that do not have any HARQ acknowledgement channels. Therefore, there is a need for a novel HARQ acknowledgement channel transmission method for guaranteeing HARQ acknowledgement performance or control channel reception while supporting backwards compatibility with legacy UEs.